I laid awake last night disassembling your motor piece by piece in my head and had a thought. From when I rebuilt my spare 2F I seem to remember there is an oil pump mounting block inside the pan that mounts to the engine block. I don't know if it's possible to bolt this on backwards or to get it out of alignment. That would offset the oil pump inlet slightly towards the cam gear. That could explain why everything seems OK until you try and seat the dizzy shaft down into the oil pump. Your existing dizzy has enough play in the shaft to allow it to go into the oil pump slightly out of alignment and still fit without binding.

This fits with your findings from last night where you ruled out the o-ring and the oil pump blade being the issue.

I think the easiest test is to drop the oil pan and see if we can re-align the oil pump and test fit a different distributor to see if it then drops in ok. If it does we can lock the pump down and put the pan back on. You may want to go ahead and replace the oil pump to be safe since you've already got the pan off.

Randy, your diagnoses of the issue was right on. It appears the block that the oil pump attaches to was indeed just a little off alignment. Fortunately the solution was very easy and didn't require the oil pan to be dropped. I loosened the two bolts that hold the block in place, removed my small cap distributor and then put in another distributor and then tightned the two bolts. I needed a long breaker bar to get them loose and I only loosened them a couple of turns. I was concerned that in loosening the bolts, I would create an oil leak so far there has not been any leak.

It took me a while to source another big cap distributor. Last week Dr. Schlegs came through with another big cap distributor (bit dirty on the inside) which went into the truck Friday evening. I put on over 120 miles this weekend and so far it is all good.

Many thanks to all who posted up on this thread and helped me figure this out.